We use the transport of amino acids across the plasma membrane of isolated cells and subcellular vesicles for the study of the phenomenon of biological membrane transport. We are investigating incompletely identified transport systems in the rat hepatocyte in primary culture and at the same time continuing such studies in the Ehrlich ascites tumor cell. The detailed description sought includes the effects of amino acid structure on the route and energization of that transport. Effects of systematic modification of the dissociation behavior of the amino acids are under study for indications as to the role of H ion flows in energization. The contribution of Na ion flows and of oxidation-reduction systems of the plasma membrane are also being studied. The effect of intensifying pH gradients is also included. Selective deletion of one of two parallel transport systems by a specific irreversible inhibitor will be attempted to see how two such systems cooperate to establish the steady state accumulation by cells.